1. Field of the Invention
The present invention relates to a material for a rail of a linear guide, which is applied to industrial machinery and devices, etc. and formed by rolling, a rail for a linear guide, a process of manufacturing a rail of a linear guide, and a linear guide.
2. Description of the Related Art
A linear guide is used to smooth the linear movements of industrial machinery and devices, etc. with good accuracy. A linear guide comprises a rail, balls and a slider. In the rail, a raceway groove is formed in each of both side surfaces, which are two surfaces parallel to each other. The balls come into rolling contact with these raceway grooves. The slider moves along the rail via the balls.
In making a rail, its contour and raceway grooves are primarily made by drawing a member of rectangular section. In a rail, heat treatment (quenching) of raceway grooves etc. is performed after a wire groove for the introduction of a wire retainer is machined in the bottom portion of the raceway grooves. After the heat treatment, a mounting hole for fixing is drilled in the rail. And after the finishing of the bottom and top surfaces of the rail by grinding, the rail is fixed to a surface plate etc. by use of the mounting hole, and finally, the raceway grooves are finished by grinding. A rail obtained by drawing has a good contour and hence it is possible to reduce the amount of grinding after that.
However, when raceway grooves are formed with good accuracy by drawing, a rail must be drawn at a plurality of steps. Moreover, production cost is high because there are many operations incidental to drawing, such as preparatory machining for introducing the material into drawing dies and anneal heat treatment. Furthermore, when the raceway grooves are finished by grinding, the rail is fixed to a surface plate by use of a mounting hole of the rail. Therefore, it is necessary that the drilling of the mounting hole be performed before the finishing process of the raceway grooves.
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-227539, there is a process of forming raceway grooves in a rail by rolling. In this case, it is possible to form raceway grooves by one rolling operation. However, a portion of the material is pushed away when raceway grooves are formed in two side surfaces of the material, which are parallel to each other, by means of roller dies, and this part of the material appears as bulges that protrude in both transverse side portions of the top and bottom surfaces, which are other two surfaces between the two side surfaces. Because the raceway grooves are formed in the side surfaces which are close to the top surface side of the rail, they protrude to a greater degree from the top surface side than from the bottom surface side.
Although the concave top surface in which the two transverse side portions bulge and the middle portion sinks has no effect on the operating accuracy of the linear guide, dust and the like are apt to accumulate on this concave top surface. For this reason, due to dust and the like which have accumulated, seals attached to the slider wear and the sealing property may sometimes deteriorate. When the top surface is chipped away until it becomes flat in order to prevent dust from accumulating on the top surface, residual stresses given during rolling are released, resulting in a decrease in the straightness of the rail and raceway grooves. For this reason, in consideration of the balance of residual stresses it is necessary to chip other portions away or separately to perform stress releasing, and production cost increases.
Even if the bottom surface is bulged a little by rolling, a rail having such a bottom surface can be used as it is if good stability is ensured during rail mounting. However, because the middle portion of this rail comes into a floating condition, controlling the tightening torque of bolts for amounting the rail becomes important in order to satisfy the operating accuracy of the linear guide. Furthermore, if the amount of protrusion is large, variations occur in the flatness of the bottom surface and the bottom surface becomes unstable during rail mounting. Therefore, it is necessary to perform flattening again by grinding or cutting. In this case, if the amount of machining is large, strains given by rolling are released and hence it becomes necessary to perform stress removing after working.